Method and apparatus for video decoding and de-interlacing

ABSTRACT

A method for video decoding in a video decoding/de-interlacing display apparatus that utilizes a storage device having four frame buffers is provided. The method includes following steps: (a) decoding video data of a next picture; (b) if the next picture is a B picture, buffering the decoded video data of the next picture into a frame buffer of the storage device not stored with a reference picture nor a present display picture nor a previous display picture; and (c) if step (b) is not applicable, buffering the decoded video data of the next picture into a frame buffer of the storage device stored with the previous display picture.

CROSS REFERENCE TO RELATED APPLICATIONS

This continuation application claims the benefit of co-pending U.S.patent application Ser. No. 10/605,517, filed on Oct. 5, 2003 andincluded herein by reference.

BACKGROUND

The invention relates to a method and an apparatus for video decodingand de-interlacing, and more particularly, to a method and an apparatusfor video decoding and de-interlacing utilizing a shared storage device.

MPEG2 is a standard that has been proposed for the digital encoding ofvideo signals. MPEG2 allows for signals to be encoded on either aninterlaced or progressive basis.

The term interlaced is used to refer to image data that is representedusing, e.g., two alternating image fields. One field of an interlacedvideo frame normally corresponds to the odd lines of pixels in the framewith the other field corresponding to the even lines of the frame.During display, the lines of one field are scanned, e.g., output to adisplay device. The lines of the second field are then scanned, so thatthey are interlaced on the display device with the lines of the firstfield. In this manner, when interlaced images are displayed, odd andeven lines of image data are updated on an alternating basis.

In the case of progressive display, image data is displayedsequentially, e.g., starting at the top left corner of an imageproceeding to the bottom right corner. Thus, in the progressive displaycase, the lines of an image are displayed or updated on a sequentialbasis without lines being skipped.

In a series of progressive video display, the positioning of thehorizontal display lines is consistent from image to image. Accordingly,each newly displayed progressive image normally will completely replacethe previous image on a display assuming that the images are the samesize. In the case of interlaced video display, each frame includes twofields which correspond to spatially different, e.g., horizontal odd andeven lines, of a display device. Accordingly, in the case of interlacedimages, each field updates only a portion of the displayed image.Because fields of an interlaced image normally correspond to images atdifferent times, merely combining fields 1 and 2 of an interlaced framecan cause blurring and other image distortions when motion is present.For this reason, conversion of interlaced images to progressive imagesnormally involves some form of motion detection and the application ofprocessing which is a function of detected motion.

Presently video images are usually encoded to be displayed as interlacedvideo. Unfortunately, most recent display devices such as monitors forcomputers and projectors are designed to display progressive displayimages.

The ability to efficiently convert between interlaced and progressiveimage formats continues to increase in importance due, in part, to theever increasing use of computers. Notably, when television scenes orother data represented as interlaced images are to be displayed on aprogressive display, they normally first have to be converted intoprogressive image data.

High speed memory is normally used in video processing applicationswhich convert between image formats. This is so that real time, or nearreal time, processing of video data can be achieved. While the cost ofmemory has dropped considerably in recent years, memory still remains asignificant cost component of many image and video processing systems.This is because a relatively large amount of memory is normally requiredfor video applications. In consumer applications and other applicationswhere cost is a concern, it is desirable to minimize the amount ofmemory required to implement an image processing system or device.

Please refer to FIG. 1. FIG. 1 shows a block diagram of a video displaysystem 100 according to the prior art. The video display system 100comprises an MPEG2 decoder 110 receiving MPEG2 coded video data streamsfrom a video source (not shown), for decoding the video data to generatedecoded video data. In order to execute the decoding function with thedecoder 110 smoothly and efficiently, a first memory is coupled to thedecoder 110 for video frame buffering. The video display system 100 thencomprises an interlaced display unit 125 coupled to the first memory120. The interlaced display unit 125 is capable of processing decodedimage data stored in the first memory 120 for further displaying on aninterlaced display (not shown). The video display system 100 alsocomprises an interlaced/progressive converter 130 coupled to theinterlaced display unit 125 for de-interlacing interlaced image datafrom the interlaced display unit 125 to generate correspondingprogressive video data and outputting the converted video data to aprogressive display (not shown) for further displaying. Again, in orderto execute the de-interlacing function with the converter 130, a secondmemory 140 is also coupled to the converter 130 for video framebuffering.

For the prior art decoder 110 to perform the decoding function towardsdifferent types of video frames, such as I picture frames, P pictureframes (together called reference picture frames), and B picture frames,the first memory 120 usually contains storage space for buffering atleast three video frames (i.e., six fields). For theinterlaced/progressive converter 130 to perform motion detection and asa result switch between intra-field and inter-field interpolation, thesecond memory 140 then requires storage space for buffering two videoframes (i.e., four fields). As a whole, to properly execute designedfunctions of the prior art video display system 100, the system 100requires a storage capacity of at least five video frames.

The prior art interlaced/progressive converter 130 usually can performmotion-adaptive de-interlacing operations with video informationincorporating not more than four video fields due to the limited storagespace of the second memory 140. That is, for each presently displayingframe, switching between intra-field and inter-field interpolation isbased on video information of at most four video fields. It is notapplicable for the prior art video display system 100 to performmotion-adaptive de-interlacing operations based on an even larger numberof fields in order to acquire a more precisely predicted display result.

As can be seen in FIG. 1, operations of the prior art video displaysystem 100 involve data exchange between the first memory 120 and thedecoder 110, between the first memory 120 and the interlaced displayunit 125, and also between the second memory 140 and theinterlaced/progressive converter 130. Therefore a total memory bandwidthrequirement of the prior art video display system 100 is considerablylarge. This is conventionally not desirable for video display systemdesign.

In order to reduce the cost as well as memory bandwidth requirement ofvideo systems that perform decoding and conversion operations, there isa need for methods and an apparatus which allow for a reduction in theamount of memory required to implement such systems. In addition, inorder to perform more well-referenced motion-adaptive de-interlacingoperations, methods and an apparatus capable of such functionality isalso desired. It is desirable that any new methods and apparatus besuitable for implementation in computer systems as well as televisionssets, set top boxes, and other video applications.

SUMMARY

It is therefore one of the objectives of the claimed invention toprovide a method and an apparatus for video decoding and de-interlacingutilizing a shared storage device having four frame buffers, to solvethe above-mentioned problem.

According to one aspect of the present invention, a method for videodecoding in a video decoding/de-interlacing display apparatus thatutilizes a storage device having four frame buffers is provided. Themethod includes: (a) decoding video data of a next picture; (b) if thenext picture is a B picture, buffering the decoded video data of thenext picture into a frame buffer of the storage device not stored with areference picture nor a present display picture nor a previous displaypicture; and (c) if step (b) is not applicable, buffering the decodedvideo data of the next picture into a frame buffer of the storage devicestored with the previous display picture.

According to another aspect of the present invention, a method for videodecoding in a video decoding/de-interlacing display apparatus thatutilizes a storage device having four frame buffers is provided. Themethod includes: (a) decoding video data of a next picture; and (b) ifthe next picture is a reference picture, buffering the decoded videodata of the next picture into a frame buffer of the storage device notstored with a last decoded reference picture nor a present displaypicture nor a previous display picture.

According to yet another aspect of the present invention, an apparatusfor video decoding and de-interlacing is provided. The apparatusincludes: a video decoder for decoding video data to generate decodedvideo data of a next picture; a storage device coupled to the videodecoder, the storage device having four frame buffers for buffering thedecoded video data of the next picture into one of the four framebuffers according to data stored in the frame buffers; aninterlace/progressive converter coupled to the storage device, forde-interlacing data stored in the frame buffers to generatecorresponding progressive video data according to a first picture and asecond picture, wherein the second picture is adjacent to the firstpicture in display order and one of the first and second pictures is apresent display picture; and a controller coupled to the video decoderand the interlace/progressive converter, for controlling data access ofthe video decoder and the interlace/progressive converter to the framebuffers of the storage device.

According to further aspect of the present invention, a method for videodecoding in a video decoding/de-interlacing display apparatus thatutilizes a storage device having four frame buffers is provided. Themethod includes: decoding video data of a next picture; buffering thedecoded video data of the next picture into one of the four framebuffers according to data stored in the frame buffers; de-interlacingdata stored in the frame buffers to generate corresponding progressivevideo data according to a first picture and a second picture, whereinthe second picture is adjacent to the first picture in display order andone of the first and second pictures is a present display picture; andcontrolling data access to the frame buffers of the storage deviceduring decoding and de-interlacing.

It is an advantage of the present invention method and apparatus toutilize a shared storage device having only four frame buffers betweenthe video decoder and the interlace/progressive converter. By performingthe present invention method, the present invention decoding andde-interlacing apparatus is capable of accomplishing advancedde-interlacing operations such as motion-adaptive de-interlacing or 3-2pull-down recovery operations with a lower hardware requirement than theprior art decoding and de-interlacing apparatus.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a video display system according to theprior art.

FIG. 2 is a block diagram of an apparatus for video decoding andde-interlacing according to the present invention.

FIG. 3 is a flow chart indicating operations of the apparatus accordingto the present invention.

FIG. 4 is a table indicating decoded video frame buffering strategyaccording to the present invention.

FIG. 5 is another table indicating decoded video frame bufferingstrategy according to the present invention.

DETAILED DESCRIPTION

Please refer to FIG. 2. FIG. 2 shows a block diagram of an apparatus 200for video decoding and de-interlacing according to the presentinvention. The apparatus 200 comprises a video decoder 210, a storagedevice 220, an interlace/progressive converter 230, and a controller240. The video decoder 210 is for decoding video data to generatedecoded video data of a next picture. The storage device 220 is coupledto the video decoder 210. The storage device 220 has four frame buffers222 for buffering the decoded video data of the next picture into one ofthe four frame buffers 222 according to data stored in the frame buffers222. The interlace/progressive converter 230 is coupled to the storagedevice 220 for de-interlacing data stored in the frame buffers 222 togenerate corresponding progressive video data. The controller 240 iscoupled to the video decoder 210 and the interlace/progressive converter230 for controlling data access of the video decoder 210 and theinterlace/progressive converter 230 to the frame buffers 222 of thestorage device 220.

Usually the de-interlaced progressive video data generated by theconverter 230 will be directly sent to a progressive display (not shown)for further displaying. However in another embodiment of the presentinvention, the de-interlaced progressive video data can also betransmitted to another dedicated storage device, such as a DRAM, forstorage. Data stored in such a storage device can be later accessed by aprogressive display for further displaying.

Please refer to FIG. 3. FIG. 3 shows a flow chart indicating operationsof the apparatus 200 according to the present invention. The presentinvention method comprises the following steps:

-   Step 300: Start.-   Step 310: Decoding video data of a next picture inputted from a    video source with the video decoder 210.-   Step 320: Judging with the controller 240 if the next picture    decoded in step 310 is a B picture or a reference picture (that is,    an I picture or a P picture). If the next picture is a B picture,    perform step 330. If the next picture is a reference picture,    perform step 350.-   Step 330: Buffering the decoded video data of the next picture into    a frame buffer 222 of the storage device 220 not stored with a    reference picture nor a present display picture nor a previous    display picture.-   Step 340: If the condition in step 330 is not applicable, buffering    the decoded video data of the next picture into a frame buffer 222    of the storage device 220 stored with the previous display picture.-   Step 350: Buffering the decoded video data of the next picture into    a frame buffer 222 of the storage device 220 not stored with the    last decoded reference picture nor the present display picture.-   Step 360: End.

Please note that as has been well know in the art, a picture (either a Bpicture, an I picture, or a P picture) referred to in the presentinvention method can be regarded as a video frame data, or a video fielddata that contributes to half of a frame data, either a top field or abottom field of the frame. For the situation when a next picture decodedis a video field, if the next picture decoded is a field (either a topor a bottom field) of a video frame whereof the other field of the samevideo frame has not yet been decoded, the present invention method shownin FIG. 3 can be directly performed, while if the next picture decodedis a field (either a top or a bottom field) of a video frame whereof theother field of the same video frame has already been decoded, the nextpicture decoded can be buffered into the frame buffer previouslybuffered with the other field of the same video frame.

By adopting the method shown in the above paragraph and in FIG. 3, thepresent invention apparatus 200 for video decoding and de-interlacing iscapable of performing MPEG type video decoding andinterlaced/progressive conversion utilizing only a shared storage devicehaving four frame buffers, instead of two separate memories dedicated tothe decoder and the converter, respectively, as shown in the prior art.

Please refer to FIG. 4 and FIG. 5 that show two tables indicatingdecoded video frame buffering strategy according to the presentinvention.

FIG. 4 lists all possible buffering situations in the frame buffers 222of the storage device 220 when the next picture decoded is a referencepicture (i.e., an I picture or a P picture). When considering the nextpicture and previous three latest decoded pictures, there are eightdifferent possible decoding sequencing situations in FIG. 4, which arethe situations of RRRR, RRBR, RBRR, RBBR, BRRR, BRBR, BBRR, and BBBR.Please note that in FIG. 4 an R represents a reference picture while a Brepresents a B picture, and the number following an R or a B indicatesthe order of said R or said B in the decoding sequence. Also note thatthe notation of B . . . B1 means a plurality of consecutive B picturesdecoded directly prior to B1.

Take the situation of R1R2R3R4 as an example, the previous three latestdecoded pictures and the next picture in the decoding sequence arerespectively R1, R2, R3, and R4. Consequently the present displaypicture R2, the previous display picture R1, and the last referencepicture R3 are stored in three of the four frame buffers 222 of thestorage device 220. Therefore, when the next picture R4 is decoded bythe video decoder 210, according to the present invention method shownin FIG. 3 (step 350) the next picture R4 can be buffered into theremaining one frame buffer 222 of the storage device 220, as shown inFIG. 4.

Another example can be shown by examining the situation of B1R2B3R4.Similar to the previous example, the previous three latest decodedpictures and the next picture in the decoding sequence are respectivelyB1, R2, B3, and R4. Also, note that a reference picture R0 appearedbefore B1 in the decoding sequence is also presented in FIG. 4.Consequently the present display picture B3, the previous displaypicture R0, and the last reference picture B2 are stored in three of thefour frame buffers 222 of the storage device 220. Therefore, when thenext picture R4 is decoded by the video decoder 210, according to thepresent invention method shown in FIG. 3 (step 350) the next picture R4can be buffered into the remaining one frame buffer 222 of the storagedevice 220, as shown in FIG. 4.

All the other situations shown in FIG. 4 can be analyzed similar to theabove two examples for those who are skilled in the art to understandthat by performing the present invention method in FIG. 3, when the nextpicture decoded is a reference picture, there will always be one framebuffer available (i.e., the remaining frame buffer) for buffering thenext picture.

In contrast to FIG. 4, FIG. 5 lists all possible buffering situations inthe frame buffers 222 of the storage device 220 when the next picturedecoded is a B picture. When considering the next picture and previousthree latest decoded pictures, there are eight different possibledecoding sequencing situations in FIG. 5, which are the situations ofRRRB, RRBB, RBRB, RBBB, BRRB, BRBB, BBRB, and BBBB. Similar to theconvention in FIG. 4, in FIG. 5 an R represents a reference picturewhile a B represents a B picture, and the number following an R or a Bindicates the order of said R or said B in the decoding sequence. Thenotation of B . . . B1 also means a plurality of consecutive B picturesdecoded directly prior to B1 (in certain situation, also directly priorto R1).

Take the situation of R1R2R3B4 as an example, the previous three latestdecoded pictures and the next picture in the decoding sequence arerespectively R1, R2, R3, and B4. Consequently the present displaypicture R2, the previous display picture R1, and the last two referencepictures R3, R2 are stored in three of the four frame buffers 222 of thestorage device 220 (note that R2 is repetitively categorized as thepresent display picture and the last two reference picture). Therefore,when the next picture B4 is decoded by the video decoder 210, accordingto the present invention method shown in FIG. 3 (step 330) the nextpicture R4 will be buffered into the remaining one frame buffer 222 ofthe storage device 220, as shown in FIG. 5.

Another example can be shown by examining the situation of B1B2B3B4.Similar to the previous example, the previous three latest decodedpictures and the next picture in the decoding sequence are respectivelyB1, B2, B3, and B4. Also, note that two reference pictures R-1 and R0appeared before B1 in the decoding sequence is also presented in FIG. 4.Consequently the present display picture B3, the previous displaypicture B2, and the last two reference pictures R-1, R0 are stored inthe four frame buffers 222 of the storage device 220. Therefore, whenthe next picture R4 is decoded by the video decoder 210, according tothe present invention method shown in FIG. 3 (step 340) the next pictureB4 will be buffered into the frame buffer 222 stored with the previousdisplay picture, which in this case is the frame buffer 222 stored withB2, as shown in FIG. 5.

All the other situations shown in FIG. 5 can be analyzed similar to theabove two examples for those who are skilled in the art to understandthat by performing the present invention method in FIG. 3, when the nextpicture decoded is a B picture, there will always be one frame bufferavailable (i.e., the remaining frame buffer or the frame buffer with B2,in accordance with different situations) for buffering the next picture.

Among all the situations defined in FIG. 4 and FIG. 5, there are twosituations RBBB and BBBB, where the next picture decoded is bufferedinto the frame buffer stored with the previous display picture. In orderto preserve information for both the present display picture and theprevious display picture at any given time, particularly in theabove-mentioned two situations, the present invention apparatus 200 iscapable of buffering a decoded next picture into a frame bufferfollowing a display operation performed on the video data presentlystored in the frame buffer. That is, the video data stored in the framebuffer has been displayed before it is substituted by the buffering dataof the decoded next picture. In other word, the buffering operation ofthe decoded next picture never precedes the displaying operation of thepresently stored video data.

In another embodiment of the present invention video decoding method aspreviously exemplified by FIG. 4 and FIG. 5, for the situationspresented in FIG. 4, where the next picture decoded is a referencepicture, instead of buffering the next decoded picture into theremaining picture the next decoded picture can also be buffered into theframe buffer 222 presently buffered with the previous display pictureaccording to the present invention method. In order to accomplish thisimplementation, An apparatus 200 having a high-speed decoder 210 or anapparatus 200 capable of buffering the decoded next picture into a framebuffer following a display operation, as described in the lastparagraph, is required.

By utilizing the present invention method depicted above, the presentinvention apparatus 200 is capable of performing motion adaptivede-interlacing operations incorporating video data of 3 through at most8 fields stored in the frame buffers 222 of the storage device 220,since the storage device 220 has four frame buffers 222 (i.e., eightfields) stored with video data of consecutive video frames.

The present invention apparatus 200 is also capable of performingrecovery operations to video data from a telecine source becauseinformation for both the present display picture and the previousdisplay picture are preserved in the frame buffers 222 of the storagedevice 220 at any given time. A movie film video data being convertedthrough the 3-2 pull-down operation shows field combining phenomenon intwo out of five video frames. That is, a video frame showing fieldcombining phenomenon incorporates two video fields extracted from twoconsecutive different film frames. By using information of a present anda previous display picture, video data from a telecine source can bedetected, recovered, and displayed by the present invention apparatus200.

In contrast to the prior art, the present invention method and apparatusutilize a shared storage device having only four frame buffers betweenthe video decoder and the interlace/progressive converter. By performingthe present invention method, the present invention decoding andde-interlacing apparatus is capable of accomplishing advancedde-interlacing operations such as motion-adaptive de-interlacing or 3-2pull-down recovery operations with a lower hardware requirement than theprior art decoding and de-interlacing apparatus.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

1. A method for video decoding in a video decoding/de-interlacingdisplay apparatus, utilizing a storage device having four frame buffers,the method comprising: (a) decoding video data of a next picture; (b) ifthe next picture is a B picture, buffering the decoded video data of thenext picture into a frame buffer of the storage device not stored with areference picture nor a present display picture nor a previous displaypicture; and (c) if step (b) is not applicable, buffering the decodedvideo data of the next picture into a frame buffer of the storage devicestored with the previous display picture.
 2. The method of claim 1,further comprising: (d) if the decoded next picture is a referencepicture, buffering the decoded video data of the next picture into aframe buffer of the storage device not stored with a last decodedreference picture nor the present display picture nor the previousdisplay picture.
 3. The method of claim 2, wherein the reference pictureis an I picture.
 4. The method of claim 2, wherein the reference pictureis a P picture.
 5. A method for video decoding in a videodecoding/de-interlacing display apparatus, utilizing a storage devicehaving four frame buffers, the method comprising: (a) decoding videodata of a next picture; and (b) if the next picture is a referencepicture, buffering the decoded video data of the next picture into aframe buffer of the storage device not stored with a last decodedreference picture nor a present display picture nor a previous displaypicture.
 6. The method of claim 5, wherein the reference picture is an Ipicture.
 7. The method of claim 5, wherein the reference picture is a Ppicture.
 8. The method of claim 5, further comprising: (c) if the nextpicture is a B picture, buffering the decoded video data of the nextpicture into a frame buffer of the storage device not stored with areference picture nor the present display picture nor the previousdisplay picture.
 9. The method of claim 8, further comprising: (d) ifstep (c) is not applicable, buffering the decoded video data of the nextpicture into a frame buffer of the storage device stored with theprevious display picture.
 10. An apparatus for video decoding andde-interlacing, comprising: a video decoder for decoding video data togenerate decoded video data of a next picture; a storage device coupledto the video decoder, the storage device having four frame buffers forbuffering the decoded video data of the next picture into one of thefour frame buffers according to data stored in the frame buffers; aninterlace/progressive converter coupled to the storage device, forde-interlacing data stored in the frame buffers to generatecorresponding progressive video data according to a first picture and asecond picture, wherein the second picture is adjacent to the firstpicture in display order and one of the first and second pictures is apresent display picture; and a controller coupled to the video decoderand the interlace/progressive converter, for controlling data access ofthe video decoder and the interlace/progressive converter to the framebuffers of the storage device.
 11. The apparatus of claim 10, whereinthe controller controls the data access of the video decoder accordingto following principles: (a) if the next picture is a B picture,buffering the decoded video data of the next picture into a frame bufferof the storage device not stored with a reference picture nor thepresent display picture nor a previous display picture; and (b) if step(a) is not applicable, buffering the decoded video data of the nextpicture into a frame buffer of the storage device stored with theprevious display picture.
 12. The apparatus of claim 10, wherein thecontroller controls the data access of the video decoder according tofollowing principles: if the next picture is a reference picture,buffering the decoded video data of the next picture into a frame bufferof the storage device not stored with a last decoded reference picturenor the present display picture nor a previous display picture.
 13. Theapparatus of claim 10, wherein the interlace/progressive converter iscapable of performing motion adaptive de-interlacing operations.
 14. Theapparatus of claim 13, wherein the interlace/progressive converterperforms the motion adaptive de-interlacing operations incorporatingvideo data of 3-8 fields stored in the frame buffers of the storagedevice.
 15. The apparatus of claim 10, wherein the apparatus is capableof performing recovery operations to video data from a telecine source.16. The apparatus of claim 10, wherein another one of the first orsecond picture other than the present display picture is a previousdisplay picture.
 17. A method for video decoding in a videodecoding/de-interlacing display apparatus, utilizing a storage devicehaving four frame buffers, the method comprising: decoding video data ofa next picture; buffering the decoded video data of the next pictureinto one of the four frame buffers according to data stored in the framebuffers; de-interlacing data stored in the frame buffers to generatecorresponding progressive video data according to a first picture and asecond picture, wherein the second picture is adjacent to the firstpicture in display order and one of the first and second pictures is apresent display picture; and controlling data access to the framebuffers of the storage device during decoding and de-interlacing. 18.The method of claim 17, wherein controlling data access comprises: (a)if the next picture is a B picture, buffering the decoded video data ofthe next picture into a frame buffer of the storage device not storedwith a reference picture nor the present display picture nor a previousdisplay picture; and (b) if step (a) is not applicable, buffering thedecoded video data of the next picture into a frame buffer of thestorage device stored with the previous display picture.
 19. The methodof claim 17, wherein controlling data access comprises: if the nextpicture is a reference picture, buffering the decoded video data of thenext picture into a frame buffer of the storage device not stored with alast decoded reference picture nor the present display picture nor aprevious display picture.
 20. The method of claim 17, wherein theinterlace/progressive converter is capable of performing motion adaptivede-interlacing operations.
 21. The method of claim 20, wherein theinterlace/progressive converter performs the motion adaptivede-interlacing operations incorporating video data of 3-8 fields storedin the frame buffers of the storage device.
 22. The method of claim 17,wherein another one of the first or second picture other than thepresent display picture is a previous display picture.